This invention relates to the field of digital color printing and, more particularly, to improving print quality by controlling migration or "bleeding" between incompatible liquid printing solutions in a printed product. The invention also is directed to alleviating physical color gamut discontinuities in printing systems, e.g. CMYK systems, that have a redundant color of ink.
Ink jet print cartridges include a plurality of orifices or nozzles, often arranged in vertical columns, for ejecting drops of ink onto the paper. For color ink jet printing, the print cartridge typically includes nozzles for ejecting cyan, magenta and yellow colored inks, called the primary printing colors, or simply "primaries." Some systems additionally include nozzles for ejecting black ink.
Printing occurs as the print cartridge traverses across the width of the paper (a "pass"). As it does so, discrete drops of ink ejected from the nozzles strike the paper or other substrate and then dry to form dots that, when viewed together, create the permanently printed image. Desired image colors are created by combining drops of ink of the primary colors where necessary. The individual dots, typically located on 1/300 inch centers, are not readily discernable to the unaided human eye so that arrays of dots can be printed to form what appear to be solid fields of a desired color.
An important consideration in printing strategies in an ink jet printing system is the intended printing medium. For example, overhead transparencies (OHT) have less affinity for absorbing ink than does a typical paper. As a result, drops of ink deposited on an OHT tend to bead rather than diffuse, as compared to drops deposited on paper. Additionally, the drops of ink deposited on OHT take longer to dry.
U.S. Pat. No. 4,748,453 (Lin et al.) discloses a method of depositing spots of liquid ink upon selected pixel centers on overhead transparencies so as to prevent the flow of liquid ink from one spot to an overlapping adjacent spot. According to that method, a line of information is printed in at least two passes so as to deposit spots of liquid ink on selected pixel centers in a checkerboard pattern, wherein only diagonally adjacent pixel areas are deposited in the same pass. On the second pass, the complementary checkerboard pattern is deposited, thereby completing deposit of ink on all of the pixels in a desired area.
Printing on paper, however, presents a different problem. Paper has an affinity for the liquid ink so that substantial absorption and diffusion of each drop of ink generally occurs. On the one hand, diffusion from one drop of ink to a drop that occupies an adjacent pixel area is helpful in achieving color mixing and obtaining a solid appearance. Along a boundary between two adjacent fields of different colors, however, such diffusion results in color bleeding across the boundary, making the boundary appear fuzzy. This is an undesirable result.
U.S. Pat. No. 5,012,257 (Lowe, et al.) discloses a two-by-two pixel ("superpixel") printing strategy to reduce bleed across color boundaries while providing good color saturation. That solution, however, effectively reduces the printer resolution, as each pixel of data is printed as a corresponding two-by-two superpixel, thereby actually printing four pixel locations. Color saturation is discussed in J. Foley, et al., COMPUTER GRAPHICS PRINCIPLES AND PRACTICE (2d.ed. Addison-Wesley, 1990) at 592.
Ink absorption can be controlled to some extent by the ink chemistry. When printing black, for example text in a letter, limiting absorption is desirable in order to provide a solid black appearance, and sharp, well-defined edges of characters. For that reason, black ink is designed to be absorbed less readily than color inks. Unfortunately, this has the effect of exacerbating bleeding where black ink touches or comes very close to color inks. Black ink is therefore said to be incompatible with color inks. Other inks may be incompatible as well. FIG. 2 illustrates the bleeding problem between a black field and an adjacent yellow field.
The incompatibility problem does not appear where composite black is used instead of true black ink, as composite black is made up of color inks. It is preferable, however, to print with a true black ink wherever possible, rather than composite black, for the following reasons:
1. True black looks better than composite black. Because of practical limitations in ink chemistry, composite black often has a colored tint to it. It might appear, for example, as greenish-black, or bluish-black. Also, the print quality of composite black is more variable over paper type, temperature, humidity and other factors than true black ink.
2. In a typical computer system, print data is sent from the host computer to the printer to control the printing of each of the four colors, CMYK, where K represents black (to avoid confusion with the color blue). If an area on a page is printed with composite black, information must be sent to the printer for the CMY inks. If the same area is printed with a black pen (true black), only data for the K ink must be sent. So use of the black pen represents a potential three-to-one reduction in data transmission between the host and the printer.
3. When printing composite black, the color pen must make three passes over the same region, the first pass putting down cyan ink, the second magenta and lastly yellow. If the same region is printed with the black pen, the black pen needs to make only one pass over the region to put down black ink. This represents a significant improvement in printing speed.
What is needed, therefore, is a liquid ink printing system that allows mixing true black and color inks within a printed page and provides for high resolution printing while controlling ink migration or "bleeding". In general, ink migration (bleed) occurs due to differences in chemical and/or physical properties of inks that must touch (or come very close to) each other on a printed page. In some cases, a substantial difference in surface tension appears to be the culprit, but other factors may contribute.
One way to avoid ink migration is to maintain at least a specified minimum separation between incompatible inks, e.g. black and color inks, on a printed page so that the inks cannot interact with one another. Methods of color separation are described in commonly-owned patent application Ser. No. 07/784,498. According to the invention of that application, black data which would be printed too close to color is instead printed as composite black (i.e. using color inks). This approach is not ideal, however, for some applications because black data "propagates" over the page as composite black, forcing use of composite black even in some areas seemingly remote from color (non-black) data. A need remains, therefore, to allow liquid ink color printing, including use of true black ink and color inks, or other combinations of otherwise incompatible inks, without unsightly bleeding between the inks.
Another problem in the prior art of digital color printing is the discontinuities that typically appear in a printing device's physical (true) color gamut. These discontinuities appear in systems having at least one redundant color, i.e. systems that have more than one physical way to produce the same logical color. By "physical color" we mean the color actually produced on a printed page, as might be observed by counting drops of C, M and Y inks. This is distinguished from a "logical color" which is image data, for example CMY data, comprising 8-bit digital values for each primary color for each pixel.